This invention relates to rolls, and more specifically, to a method of manufacturing pulverizer rolls of the type that are intended to be used in bowl mills for purposes of effecting the pulverization therein of material such as, for example, coal.
An essential component of any steam generation system of the type, which utilizes pulverized coal as a fuel, is the apparatus in which the coal is pulverized so as to render it suitable for such usage. Although the prior art is known to have employed various types of apparatus for purposes of accomplishing coal pulverization, one form of apparatus in particular, which has frequently been used for this purpose, is that commonly referred to as a bowl mill by those in the industry. The bowl mill obtains its name principally from the fact that the pulverization, i.e., grinding, of the coal that takes place therewithin occurs on a grinding surface, which in configuration somewhat resembles a bowl.
By way of illustration, reference may be had to U.S. Pat. No. 3,465,971, which is assigned to the same assignee as the present invention, for a showing of a prior art form of bowl mill. This patent contains a teaching of both the nature of the construction and the mode of operation of a bowl mill that is suitable for use for purposes of effecting the pulverization of the coal that is used to fuel a coal-fired steam generator. As taught by this patent, the essential components of such a bowl mill are a body portion, i.e., housing, within which a grinding table is mounted for rotation, a plurality of grinding rolls that are supported in equally spaced relation one to another in a manner so as to coact with the grinding table such that the coal disposed on the surface of the grinding table is capable of being ground, i.e., pulverized, by the rolls, coal supply means for feeding to the surface of the grinding table the coal that is to be pulverized in the bowl mill, and air supply means for providing to the interior of the body portion the air that is required for the operation of the bowl mill.
In order to satisfy the demands of a coal-fired steam generation system of conventional construction for pulverized coal, a plurality of bowl mills of the type shown in the aforereferenced patent are commonly required to be employed. Further in this regard, it is noted that the individual capacity of each of these bowl mills may range up to a capacity of one hundred tons of pulverized coal per hour. In addition to possessing the capability of operating at this maximum capacity, these bowl mills must also have the ability to operate at less than full capacity, i.e., at some percentage thereof, e.g., 25%, 50%, 75%, etc. Lastly, these bowl mills must be capable of use with a variety of types of coal that possess different grinding characteristics.
In accord with the mode of construction illustrated in the aforereferenced patent, the grinding rolls are each designed to be mounted on a shaft-like member whereby the grinding rolls are capable of movement relative to the surface of the grinding table. Accordingly, in order to effect the assembly of the grinding roll with the aforesaid shaft-like member, the former preferably has a through passage provided through the center thereof. This through passage enables the shaft-like member to be positioned therewithin in mounted relation thereto. Therefore, in order to facilitate the task of providing the grinding roll with such a through passage, it is desirable that the grinding roll be made of a material that is characterized by its easy machinability, i.e., a relatively soft material such as gray iron.
In contradistinction to the desirability of making the grinding roll from a relatively soft material, there also exists a requirement that at least the external surface of the grinding roll be of a material characterized by good wear-resistant qualities. The reason for this is that in the course of effecting the pulverization of material with these grinding rolls, the latter are of necessity subjected to a harsh abrasive action by virtue of the nature of the material being pulverized as well as by virtue of the manner in which the pulverization takes place. The result, therefore, is that the grinding rolls are susceptible to being rendered unusable because of excessive wear in a relatively short period of time, i.e., the rolls are found to have a relatively short operating life. Obviously, such a result is to be avoided, if possible. In this context, it should be noted that the wear which grinding rolls employed in bowl mills actually experience is influenced principally by the grinding characteristics of the material being pulverized as well as by the productive output of the bowl mill, i.e., the volume of material that is being pulverized within the bowl mill in a given period of time.
When the external surface of the pulverizer, i.e., grinding, roll becomes sufficiently worn to preclude any further use thereof for purposes of effecting the pulverization of material therewith, the remaining portions of the roll are normally still functional. That is, but for its worn external surface, the grinding roll would still be capable of being used. Thus, from the standpoint of achieving economies of manufacture, it would be desirable, if possible, to effect a resurfacing of the external surface of the grinding roll such that the latter might once again be utilized for purposes of pulverizing material rather than to have to replace the worn grinding roll with a totally new grinding roll. In this regard, one would normally expect that the cost of reconditioning, i.e., resurfacing the external surface of a worn grinding roll would be significantly less costly than manufacturing a totally new grinding roll.
Apart from the relative cost of resurfacing the external surface of a worn grinding roll vis-a-vis that of manufacturing a totally new grinding roll, there is also the matter of the operating life achievable with a resurfaced worn grinding roll as contrasted to that obtainable with a new grinding roll. That is, in order to be economically feasible, it is desirable that the operating life of a resurfaced worn grinding roll be approximately equivalent to or greater than that which one would expect from a new grinding roll. Namely, any savings achieved from utilizing a resurfaced worn grinding roll as opposed to employing a new grinding roll should not be dissipated by virtue of the fact that the operating life of the former is such that several resurfacings thereof would be required in order to achieve an operating life with the worn grinding roll which approximates that obtainable from the use of a totally new grinding roll. In summary, if in the interest of obtaining comparable operating lives, multiple resurfacings of the worn grinding roll would be required, the cost thereof would probably equal or exceed the cost of manufacturing a totally new grinding roll whereby the advantages accruing from reusing a worn grinding roll vis-a-vis replacing the latter with a new grinding roll would not be realized.
It has been known in the prior art to resurface a worn grinding roll. Moreover, the results obtainable therefrom have proven to be generally satisfactory. More commonly, the resurfacing has taken the form of providing the external surface of the worn grinding roll with a suitably dimensioned layer of weld metal. In view of the prior art's experience with the resurfacing of a worn grinding roll, attempts have been made to provide the external surface of new grinding rolls with a hard facing. Unfortunately, such attempts when applied to new grinding rolls have not met with a great deal of success heretodate. The motivation behind these attempts resides largely in the desire to effectively extend the operating life of new grinding rolls. A major reason why the attainment of a longer operating life for new grinding rolls is sought is that it would enable one to extend the point in time at which it becomes necessary to shut down the bowl mill for purposes of enabling the removal therefrom of the worn grinding roll, and the replacement thereof with an unworn grinding roll. In this regard, mention is once again made of the fact that a plurality of bowl mills are commonly employed to provide the required amount of pulverized coal to a coal-fired steam generator, and that each of these bowl mills normally embodies three grinding rolls that are each susceptible to having to be removed and replaced as they become worn. Also, there is the matter of the time and effort and the cost associated therewith that needs to be expended in the course of effecting such removal and replacement of a worn grinding roll. Obviously, therefore, it one were able to reduce the frequency with which grinding rolls become sufficiently worn as to require replacement, cost savings could be realized in terms of the time and effort required to be expended to effect such replacement. A need has, thus, been shown to exist in the prior art for a new and improved method of manufacturing pulverizer rolls which would be capable of producing pulverizer rolls that would be characterized by the fact that the latter possessed a significantly longer operating life than that commonly found to be obtainable from a newly manufactured pulverizer roll that has been manufactured in accordance with heretofore known prior art methods.
It is, therefore, an object of the present invention to provide a new and improved method of manufacturing a pulverizer roll.
It is another object of the present invention to provide such a method whereby the pulverizer roll manufactured in accordance therewith is primarily suitable for embodiment in a bowl mill for use therein to effect the pulverization of a material such as coal.
It is still another object of the present invention to provide such a method wherein the pulverizer roll manufactured in accordance therewith is characterized by its relatively long operating life.
A further object of the present invention is to provide such a method wherein the pulverizer roll manufactured in accordance therewith achieves its relatively long operating life by virtue of having a hard facing applied in a novel manner to the external surface of the pulverizer roll.
A still further object of the present invention is to provide such a method wherein the pulverizer roll manufactured in accordance therewith achieves its relatively long operating life by virtue of having a layer of weld metal applied in a novel manner to the external surface of the pulverizer roll.
Yet another object of the present invention is to provide such a method wherein the pulverizer roll manufactured in accordance therewith is characterized by the fact that the external surface thereof is shaped in accordance with the predicted wear characteristics that the pulverizer roll is expected to experience based on the nature of the use to which the pulverizer roll is intended to be put such that the contour thereof replicates that of a worn pulverizer roll.
Yet still another object of the present invention is to provide such a method which is relatively easy to employ, and which enables economies of manufacture to be realized through the use thereof.